The idea that Forrest jr isn’t Forrest’s kid is some cynical stuff lol. I remember when I first read that I was thinking, damn is there really anything to imply that in the movie or are you guys just assuming.
I'm going to level with you. Jenny had a lot of issues but she was not an extremely malicious person (from what I recall of the movie- which, granted, I haven't seen in years). And I don't think she exploited Forrest or was looking to work some angle on him. I think she genuinely had affection for him.
Is it beyond the realm of possibility that if Forrest Jr. was not in fact Forrest's kid, she would have just talked with Forrest, told him she was dying and entreated him to take care of her child because she knows he will treat him well and give him a good life. She knows the caliber of guy Forrest is. He would do that without question.
She would not need to concoct some ruse whereby she named her kid Forrest Jr. deceptively to entrap a rich, slow individual into taking care of her child from another relationship.